Bacteria of the genus Chlamydia (psittacosis-trachoma organisms) will be grown in cultures of mouse fibroblasts (L cells). These problems will be studied: 1. Isolation and characterization of conditional lethal mutants of Chlamydia psittaci. 2. Isolation and identification of inclusions of C. psittaci and Chlamydia trachomatis and study of the properties of inclusion membranes and membrane-bound chlamydial cells. 3. Elucidation of the mechanisms whereby high multiplicities of C. psittaci and C. trachomatis are immediately toxic to L cells and other host cells in culture and to assess the significance of this toxicity in explaining damage to host cells in natural infections with chlamydiae.